At very high frequencies, 30 to 300 GHz for millimeter wave frequency band, typical integrated circuit transmission lines, such as microstrip or coplanar waveguide, become very lossy due to conductor and dielectric losses, and metal and substrate surface irregularities which can cause unwanted reflections and radiation. At these high frequencies, dielectric waveguides, of which there are a number of different forms provide a lower loss alternative to signal routing.
Conventional dielectric waveguide switches require a transition from the dielectric waveguide to a transmission line which leads to a localized switch circuit. Typical transmission lines have a metal strip on the top side of the circuit substrate and a metal ground on the bottom of the circuit substrate, or coplanar waveguide which has a signal strip on the top side of the substrate and two metallic grounds also on the substrate top-side which are separated on each side of the strip by a gap which is determined by the desired characteristic impedance of the line. These transitions are typically necessary to connect the image guide to sources, mixers, amplifiers, and switching, but they degrade the overall performance of the image guide system through parasitic reflections and radiation which increase as the frequency of the system increases.
At very high frequencies, these transitions and transmission lines add RF loss to the overall dielectric waveguide circuit. So, at very high frequencies, 30 Ghz and up, switches tend to be either very lossy or narrow band. What is needed is a high frequency switch that provides signal switching without having to remove the signal from the dielectric waveguide. Also, what is needed is a means to avoid the RF losses associated with metallic transmission lines at higher frequencies. Furthermore, what is needed is a device that does not require a transition from dielectric waveguide to printed circuit transmission line. This is particularly true in high frequency applications.
One alternative approach utilizes an image guide coupler. In this approach, a ferrite is placed between the image guides along the coupling region as disclosed in an article by P. Kwan and C. Vittoria, entitled “Scattering Parameters Measurement of a Nonreciprocal Coupling Structure,” in IEEE Trans. Microwave Theory Technique, Vol. 41, No. 4, April 1993, pp. 652–657. A magnetic field bias applied to the ferrite controls the coupling between the image lines. Thus, the coupling coefficient is modified by an external applied magnetic field bias on the ferrite for isolators, filters, modulators, switches, and phase shifters. With appropriate external applied magnetic field bias on the ferrite, the four port device prior art can be made into an image guide switch.
With such an approach, however, there are several problems. One problem is that ferrites become lossy at high frequency. What is need is a high frequency switch capable of providing low loss. Another problem is that ferrites are not easy to integrate into monolithic structures. Thus, there is a need for a switch capable of easy integration into monolithic integrated circuit structures.